AN2242 APPLICATION NOTE - STMicroelectronics€¦ · Reference design: high performance, L6668-based flyback converter for Set-Top boxes and PVRs Introduction This document describes

Rev 1.0AN2242/1205 1/31

31

AN2242APPLICATION NOTE

Reference design: high performance, L6668-based flybackconverter for Set-Top boxes and PVRs

IntroductionThis document describes a reference design of a 40W Switch Mode Power Supply dedicated to Set-Top box application. The board accepts wide range input voltage (90 to 265Vrms) and delivers 4 outputs. It is based on the new controller L6668, working in PWM fixed frequency current mode. High efficiency and low standby consumption are the main characteristics of this board. Such features, coupled with the minimal part count required and the global solution low cost approach, makes it an ideal solution for powering digital consumer equipment, meeting worldwide standby rules.

Figure 1. EVAL6668-STB Demo Board, Described In This Application Note

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Contents

1 Main Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Circuit Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Cross Regulation and Standby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Functional Checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1 Start-up Behaviour at Full Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2 Wake-up Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3 Power-down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.4 Short-Circuit Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.5 Over Voltage Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 Conducted Noise Measurements (Pre-Compliance Test) . . . . . . . . . . . . 21

6 Thermal Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

7 Part List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

8 PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9 Transformer Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

9.1 Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

9.2 Mechanical Aspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

9.3 Manufacturer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

10 Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

AN2242

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Figures

Figure 1. EVAL6668-STB Demo Board, Described In This Application Note. . . . . . . 1Figure 2. Electrical Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Figure 3. Vin = 115 Vrms - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 4. Vin = 230 Vrms - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 5. Vin = 265 Vrms - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 6. Vin = 115 Vrms - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 7. Vin = 220 Vrms - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 8. Vin = 230 Vrms - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 9. Input Power vs. Mains Voltage During Standby . . . . . . . . . . . . . . . . . . . . 13Figure 10. Pin at 230vac vs. Iout 5V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 11. Vin = 230 Vrms - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 12. Vin = 90 VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 13. Vin = 265 VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 14. at 115 VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 15. at 230 VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 16. at 115 VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 17. at 230 VAC - 50Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 18. 12V OUTPUT SHORT at 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 19. 3.3V OUTPUT SHORT at 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 20. 3.3V OUTPUT SHORT at 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 21. 5V OUTPUT SHORT at 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 22. 1.8V OUTPUT SHORT at 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 23. 1.8V OUTPUT SHORT at 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 24. OVP at 115 VAC - 60Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 25. Conducted Noise Measurements - Phase A . . . . . . . . . . . . . . . . . . . . . . . 21Figure 26. Concucted Noise Measurements - Phase B . . . . . . . . . . . . . . . . . . . . . . . 21Figure 27. 115Vac-Max Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 28. 230Vac-Max Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 29. Silk Screen -Top Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 30. Silk Screen -Bottom Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 31. Copper Tracks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 32. Transformer Electrical Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 33. Winding Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

AN2242

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Tables

Table 1. Output Voltages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Table 2. Output Voltage Measurement at Full Load . . . . . . . . . . . . . . . . . . . . . . . . 11Table 3. Output Voltage Measurement at HDD SPIN-UP . . . . . . . . . . . . . . . . . . . 11Table 4. Output Voltage Measurement at Reduced, W/O HDD . . . . . . . . . . . . . . . 12Table 5. Output Voltage Measurement at Minimum Load. . . . . . . . . . . . . . . . . . . . 12Table 6. Output Voltage Measurement at Standby Load . . . . . . . . . . . . . . . . . . . . 13Table 7. Part List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Table 8. Winding Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

AN2242 1 Main Characteristics

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1 Main Characteristics

The main characteristics of the SMPS are listed here below:

INPUT VOLTAGE:

- Vin: 90 - 264 Vrms

- f: 45-66Hz

OUTPUT VOLTAGES:

Table 1. Output Voltages

STANDBY: Below 1W with 5V at 50mA residual load

SHORT CIRCUIT PROTECTION: On all outputs, with auto-restart at short removal

PCB TYPE & SIZE: Single Side 70um (2-Oz), CEM-1, 150 x 75mm

SAFETY: Acc. to EN60065, creepage and clearance minimum distance 6.4mm

EMI: Acc. to with EN50022-Class B

Vout IoutMAX IoutMIN PMAX STABILITY NOTES

1.8V 1.73A 0.20A 3.1W ±7%Dedicated to digital circuitry and to 1.2V local post regulators

3.3V 0.5A 0.03A 1.65W ±5%Dedicated to analog peripherals and 2.5V regulators

5V 2.4A 0.3A 12W ±10% Dedicated to HDD and 5V circuitry

12V1.9Avg2.9Apk

0.05A 34.8W ±8%

Dedicated to HDD, SCART, LNBP21 for satellite STB.Average load is 1.9A, 2.9A is dedicated to HDD spin-up

POUT(W)=40WAVG / 51Wpk

1 Main Characteristics AN2242

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Figure 2. Electrical Diagram

g

R20

1K8

R23

1K0

R24

1K0

C37

3N3

C32

100N

C24

220u

F-16

V Y

XF

1 2

4 3

U3

SFH

617A

-4

C9

470u

F-25

V Y

XF

L2

2.7u

H

C28

100u

F-25

V

+3V

3

@0.5

A

C11

100u

F-25

V Y

XF

D9

STP

S1L

60A

D12

1N58

21

C35

2200

uF-1

6V Y

XF

C29

100u

F-25

V

+1V

8

@1.7

A

L3

2.7u

H

C22

1000

uF-1

6V Y

XF

C27

100u

F-25

V

+5V

@2.4

A

L4

22uH

C10

470u

F-25

V Y

XF

C14

100N

C15

100N

C17

100N

C12

100N

+12

V

R4

4K7

R25

3K3

R26

1K0

C21

10N

-400

V

R7

33K

-2W

R1

NT

C_1

0R S

236

F1 FUSE

2A

C18

220N

-X2

90-2

64V

rms

C20

82uF

-400

V

C30

2N2

Q1

STP

4NK

60Z

FP

R8

33R

R17

82R R

161K

0

C34

220P

C16

100N

C31

100N

R15

6K2

C33

10N

C19

47uF

-50V

R5

470K

R6

330K

R22

27K

R21

3K9

C36

470P

R12

10K

R13

36K

R19

0R47

R18

0R47

341

2

L1

HF2

430-

203Y

1R0-

T01

TD

K

D5

BA

V10

3

4

1

3

2

D1

2W06

G

D6

BA

V10

3

D7

STT

H1L

06U C5

2N2

- Y

1

T1

SRW

28L

EC

-E01

1 2

J1 INP

UT

CO

NN

.

HS1

C39

100N

C38

100N

1 3 5 6

@1.9

A-2

.9A

pk

L5

2.7u

H

D2

STP

S8H

100F

P

C23

1000

uF-1

6V Y

XF

R10

10K

C7

1N0

R2

3R9

R14

30K

R27

5K6

SKIP

AD

J9

ISE

N12

HVS(nc)2

HV1 O

UT

4

VR

EF

8

RC

T16

VC

C5

S_C

OM

P15

PFC_STOP14

ST-B

Y13

GN

D3

SS11

N.C

.6

OVP7

COMP10

U2

L6668

D10

STP

S10L

60FP

1 2 3 4 5 6 7 8J2 O/P

CO

NN

+1V

8

+5V

+12

V

+3V

3

D13

LL

4148

Q2

BC

847B

Q3

BC

847B

R30

270R

R29

10K

R28

1K

R34

1K0

R33

1K0

R32

1K0

R35

1K0

78910

11

12

U4

TS2

431I

LT

C41

2

HS2

HS3

L28

5

C41

10uF

-50V

D16

LL

4148

R36 3K

3

C40

10uF

-50V

Q5

BC

847B

Q4

BC

857B

DIS

AN2242 2 Circuit Description

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2 Circuit Description

The topology used in this schematic is a classical flyback, working in continuous and discontinuous conduction mode with fixed frequency, achieves the best tradeoff between peak/rms current ratio and the output capacitors' size. The nominal switching frequency, 65kHz, has been chosen to get a compromise between the transformer size and the harmonics of the switching frequency, optimizing the input filter size and its cost. The input EMI filter is a classical Pi-filter, 1-cell for differential and common mode noise. A NTC limits the inrush current produced by the capacitor charging at plug-in. The MOSFET is a standard and cheap 600V-2Ω max, TO-220FP, needing a small heat sink. The transformer is a layer type, using a standard ferrite type EER28L. The transformer, designed according to the EN60065, is manufactured by TDK. The reflected voltage is 70V, providing enough room for the leakage inductance voltage spike with still margin for reliability of the MOSFET. The network D7, R7, C21 clamps the peak of the leakage inductance voltage spike. The controller is the new L6668, integrating all the functionalities needed to control an SMPS with high performance and minimum component count, offering the maximum flexibility. A new functionality embedded in the device is a high-voltage current source used at start-up that draws current from the DC bus and charges the capacitor C19. After the voltage on C19 has reached the L6668 turn-on threshold and the circuit starts to operate, the controller is powered by the transformer via the diode D5. After the start-up, the HV current source is deactivated, saving power during normal operation and allowing very good circuit efficiency during standby. The control system is Current Mode, so the current flowing in the primary is sensed by R18 and R19 and is then fed into pin #12 (Isen). R5 and R6 are also connected to pin #12 (Isen). Their purpose is to compensate the power capability change vs. the input voltage. The resistor R27 connected between pin #12 (Isen) and pin #15 (S_Comp) provides the correct slope compensation to the current signal, necessary for the correct loop stability. The circuit connected to pin #7 (DIS) provides over voltage protection in case of feedback network failures and open loop operation. An internal comparator senses this pin voltage and in case its threshold is exceeded the L6668 stops operating and reduces its consumption. To definitely latch this state an internal circuitry of the L6668 monitors the Vcc periodically reactivates the HV current source to supply the IC. After OVP detection and Disable intervention the controller operation can be resumed only after disconnecting the mains plug. The switching frequency is programmed by the RC connected to pin #16 (RCT) and in case of reduced load operation the controller can decrease the operating frequency via the pin #13 (ST-BY) and resistor R15, proportionally to the load consumption. The resistor dividers R13 and R14 connected to pin #9 (SKIPADJ) allow to set the initial L6668 threshold to burst mode functionality when the power supply is lightly loaded. Additional functions embedded in the L6668 are the programmable soft-start and a 5V reference available externally. The output rectifiers have been selected according to the calculated maximum reverse voltage, forward voltage drop and power dissipation. The rectifiers for 5V and 12V outputs are Schottky, low forward voltage drop type, hence they dissipate less power with respect to standard types. A small heat sink for both devices is required, as indicated on the BOM. The other two output rectifiers are Schottky too but with a smaller package. The snubber made up of R2 and C7 damps the oscillation produced by the diode D2 at MOSFET turn-on. The output voltage regulation is performed by secondary feedback on the 12V, 5V and 3.3V output, while for the 1.8V output the regulation is achieved by the transformer coupling. The feedback network uses a TS2431 driving an optocoupler, in this case a SFH617A-4, ensuring the required insulation between primary and secondary. The opto-transistor drives directly the COMP pin of the L6668. A small LC filter has been added on all outputs in order to filter the high frequency ripple without increasing the output capacitors and a 100nF capacitor has been placed on each output, very close to the output connector solder points, to limit the spike amplitude.

2 Circuit Description AN2242

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Here follow some waveforms during normal operation at full load:

The pictures above show the drain voltage and current signal on pin #12 at the nominal input mains voltage during normal operation at full load. The Envelope acquisition of the scope provides for the possibility to observe the modulation of the two waveforms, due to the mains input voltage ripple at twice the line frequency.

Figure 5. The drain voltage waveforms and the measurement of the peak voltage at full load and maximum input mains voltage are shown. The maximum voltage peak in this condition is 524V, which ensures a reliable operation of the power MOSFET with a good margin against the maximum BVDSS. The operation during the hard-disk spin-up when the SMPS delivers the peak power to the loads, brings the drain voltage peak at 540Vpk.

Figure 5. Vin = 265 Vrms - 50Hz

Figure 5. and Figure 6. depict the most salient controller IC signals. In all pictures it is possible to distinguish clean waveforms free of hard spikes or noise that could affect the controller

Figure 3. Vin = 115 Vrms - 60Hz Figure 4. Vin = 230 Vrms - 50Hz

CH1: DRAIN VOLTAGECH2: DRAIN CURRENT - VPIN12 (Isen)



AN2242 2 Circuit Description

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correct operation. More precisely, in Figure 5. and Figure 6. the waveforms during normal operation at max load and both nominal input mains voltage measured on current sense pin, gate drive, comp and oscillator pin have been captured. It is possible to notice that the circuit works in continuous over the entire input mains range.. Besides, the signal on pin #12 (Isen), has a different offset and similar peak for the effect of R5 and R6. Their purpose is in fact to compensate the over current set point variation with the input voltage, which instead has to be almost constantover the entire input voltage mains range. Of course, the oscillator waveform does not change in the two pictures.

In Figure 8. channel 4 shows the slope compensation signal, coming from pin #15. This pin provides a voltage ramp during the MOSFET's ON-time, which is a repetition of the oscillator saw tooth and is dedicated to implementing additive slope compensation on current sense. This is needed to avoid the sub-harmonic oscillation that arises in all peak-current-mode-controlled converters working in continuous conduction mode with a duty cycle close to or exceeding 50%, as in this circuit. Besides, only a resistor is needed to implement slope compensation, and since the voltage ramp is delivered during the ON-time only, the correct operation of the control circuit is ensured even at light load. The slope compensation signal delivered only during ON-time in fact prevents perturbations on the current sense due to the injection of the residual part of the oscillator saw tooth that could affect the control circuit, as it happens using the oscillator total saw tooth for slope compensation.

Figure 6. Vin = 115 Vrms - 60Hz Figure 7. Vin = 220 Vrms - 50Hz

CH1: VPIN12 - ISENCH2: VPIN4 - OUTCH3: VPIN16 - RCTCH4: VPIN10 - COMP

CH1: VPIN12 - ISENCH2: VPIN4 - OUTCH3: VPIN16 - RCTCH4: VPIN10 - COMP

2 Circuit Description AN2242

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CH1: VPIN12 - ISENCH2: VPIN4 - OUTCH3: VPIN16 - RCTCH4: VPIN15 - S_COMP

AN2242 3 Cross Regulation and Standby

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3 Cross Regulation and Standby

The following tables show the output voltage measurements and the overall efficiency of the converter measured at different input voltages. All the output voltages have been measured on the output connector.

Table 2. Output Voltage Measurement at Full Load

Table 2. lists the output voltage measurements at both nominal mains range: all voltages are within the tolerance given in the specification. The efficiency measured is very good for this kind of SMPS, thanks to the absence of post regulators.

Table 3. shows the same measurement done during the HDD spin-up, which means higher current from the 12V output, while the other output currents are not changed.

Table 3. Output Voltage Measurement at HDD SPIN-UP

As clearly visible in this table, the output voltages are still within the given tolerance. The heavier load does not affect efficiency. Please note that that the total output power of Table 3. is the so-called "electrical power", not the "thermal power". Therefore, the circuit has been designed to deliver the "electrical power" for a short time only - typically the HDD spin-up has

at 115Vac - 60Hz at 230Vac - 50Hz

Voltage [V] Current [A] Deviation Voltage [V] Current [A] Deviation

11.70 1.9 -2.50% OK 11.6 1.9 -3.33% OK

1.81 1.7 0.56 OK 1.82 1.7 1.11% OK

4.82 2.4 -3.60% OK 4.82 2.4 -3.60% OK

3.28 0.5 -0.61% OK 3.28 0.5 -0.61% OK



11.51 2.9 -4.08% OK 11.46 2.9 -4.50% OK

1.81 1.7 0.56% OK 1.82 1.7 1.11% OK

4.77 2.4 -4.60% OK 4.80 2.4 -4.00% OK

3.3 0.5 -0.00% OK 3.29 0.5 -0.30% OK

Vin= 115Vac

lin= 0.75Arms

Pin= 49W

EFF.= 78.6%

Vin= 230Vac

lin= 0.44Arms

Pin= 47.6W

EFF.= 80.6%

Pout=38.52W Pout=38.34W

Vin= 115Vaclin= 0.93Arms

Pin= 64.5WEFF.= 76.8%

Vin= 230Vaclin= 0.55ArmsPin= 61.2W

EFF.= 80.9%


3 Cross Regulation and Standby AN2242

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duration of 1 second - but it cannot be delivered significantly longer because, from the thermal point of view, the circuit can manage the full load only with the required reliability. The board has been designed to power a digital decoder equipped with a hard disk drive but it can be used even for powering a Set-Top box without the Hard disk drive. The measurements shown in Table 4. are relevant to a load condition typical of a satellite Set-Top box. The 12V and 5V loads have been reduced with respect to the previous measurements.

Table 4. Output Voltage Measurement at Reduced, W/O HDD

At minimum load the power supply is still capable of keeping the output voltages regulated within the specification, as indicated by the measurements in Table 5. Additionally, the auxiliary voltage has been measured too: note that the voltage powering the IC has a good margin with respect to the L6668 turn-off voltage.

Table 5. Output Voltage Measurement at Minimum Load



11.86 1.1 -1.17% OK 11.76 1.1 -2.00% OK

1.79 1.7 -0.56% OK 1.81 1.7 0.56% OK

4.85 1.8 -3.00% OK 4.9 1.8 -2.00% OK

3.27 0.5 -0.91% OK 3.26 0.5 -1.21% OK

Pout= 26.45 W Pout= 26.46 W



11.72 0.05 -2.33% OK 11.65 0.05 -2.92% OK

1.80 0.2 0.00% OK 1.80 0.2 0.00% OK

4.77 0.3 -4.600% OK 4.78 0.3 -4.40% OK

3.35 0.03 1.52% OK 3.34 0.03 1.21% OK

Vin= 115Vaclin= 0.48ArmsPin= 33.1WEFF.= 79.9%

Vin= 230Vaclin= 0.29ArmsPin= 33.4W

EFF.= 79.2%


Vin= 115Vac

lin= 0.082Arms

Pin= 3.6W

Vaux=1.5V

EFF.= 68.8%

Vin= 230Vac

lin= 0.05Arms

Pin= 4.2W

Vaux=11.4V

EFF.= 59.0%


AN2242 3 Cross Regulation and Standby

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Table 6. Output Voltage Measurement at Standby Load

In Table 6. the output voltage measurements during standby operation are shown. Even in this load condition the circuit is able to regulate the output voltages within the required tolerance.

Figure 9. Input Power vs. Mains Voltage During Standby

It is clearly visible that, while delivering the required standby load, (5V at 50mA, 1.8V, 3.3V and 12V at 0mA) the input power consumption is below 800mW at both the nominal input voltage ranges and remains below 900mW at 265Vac. Figure 10. represents the input power variation as a function of the 5V current.

During the standby operation the circuit works in burst mode, thus the number of switching cycles decreases and, therefore it brings to an equivalent continuous switching frequency reduction. This minimizes all frequency-related losses and maximizes the efficiency with respect to other controllers without this feature. The L6668 enters automatically in burst mode when its internal circuitry detects a light load by monitoring the voltage on pin #10 (Comp). When it falls by 50mV below the threshold programmed via the divider connected at to pin #9 (SKIPADJ), the IC is disabled and its consumption is reduced to minimize the Vcc capacitor discharge. The soft-start capacitor is not discharged. The control voltage now will increase as a result of the feedback reaction to the energy delivery stop, the threshold will be exceeded and the IC will restart switching again. In this way the converter works in burst-mode with a constant

at 115Vac - 60Hz at 230Vac


11.80 0 -1.67% OK 11.70 0 -2.50% OK

1.92 0 6.67% OK 1.92 0 6.67% OK

4.71 0.047 -5.80% OK 4.73 0.047 -5.40% OK

3.36 0 1.82% OK 3.36 0 1.82% OK


Pin=0.51WVaux=9.68V

EFF.= 43.5%


Pin=0.75WVaux=10V

EFF.= 29.8%


00.10.20.30.40.50.60.70.80.9

1

Mains voltage

Inp

ut p

ow

er

Pin= [W] 0.49 0.51 0.75 0.88

Vaux= [V] 9.67 9.68 10 10.04

90Vac 115Vac 230Vac 265Vac

3 Cross Regulation and Standby AN2242

14/31

peak current defined by the disable level applied at to pin #9. This functionality is well visible in Figure 11., where the most significant waveforms are depicted during the standby operation. It is possible to check that the circuit stops switching as soon as the COMP pin voltage is equal to the SKIPADJ voltage and restarts to operate as soon as the threshold returns at the same level with the mentioned voltage histeresys. In the above table also the auxiliary voltage measurement is shown. At minimum load the value is still enough to ensure correct operation of the L6668 with margin. During standby operation the output voltages are only affected by a slight ripple at burst frequency, due to the operation of the internal control comparator. The ripple peak to peak measured in worst case at 265Vac on the 5V output was 68mV.

Figure 10. Pin at 230vac vs. Iout 5V


Pin @230vac vs. Iout 5V

0

0.2

0.4

0.6

0.8

1

1.2

1.4

20 30 40 50 60 70 80Iout 5V [mA]

Pin

@23

0vac

[W]

CH1: VDRAINCH2: VPIN10- COMP

CH3: VPIN16 - RCTCH4: VPIN9 - SKIP_ADJ

AN2242 4 Functional Checking

15/31

4 Functional Checking

4.1 Start-up Behaviour at Full Load

Figure 12. and Figure 13. capture the rising slopes at full load of output voltages at minimum and maximum input mains voltage. As shown in the pictures, the rising times are constant and there is no difference between the rise time of the output voltages. To avoid problems at start-up, this characteristic is quite important when the loads are a microprocessor and its peripherals, as in our case. Just a negligible overshoot is present at both mains voltages, without consequences for the supplied circuitry. The soft start time can be programmed via the soft-start capacitor connected to pin #11 of the L6668.

4.2 Wake-up Time

The wake-up time is the time needed for the power supply to deliver the nominal output voltages once it has been plugged-in or, if there is any, the mains switch has been closed. Generally, with wide mains power supplies using passive solutions for start-up like the largest part on the market, it is difficult to find a good compromise between the wake-up time at low mains, the dissipation at high mains and the circuit complexity. The following figures show the waveforms with the wake-up time measurements at both nominal input mains. The measured wake-up time at 115Vac and 230Vac is 750millisecond, which is a common value for this kind of power supplies. Thus, thanks to high voltage circuitry integrated in the L6668, the wake-up time is perfectly constant vs. the input voltage, which can be achieved without any external component or special circuitry. Moreover, as soon as the IC has started, the HV current source is switched off, saving power that would affect the efficiency and the standby performance of the SMPS otherwise.

Figure 12. Vin = 90 VAC - 60Hz Figure 13. Vin = 265 VAC - 50Hz

CH1: +12 VoutCH2: +5 Vout

CH3: +3.3 VoutCH4: +1.8 Vout

4 Functional Checking AN2242

16/31

To avoid spurious start-up attempts when power supply is plugged in with abnormal, lower voltage mains, the HV current source has a turn-on threshold so that it is not activated if the input voltage is lower than the Start voltage (typ. value is 80V).

As visible in Figure 14. and Figure 15. captured at the nominal mains voltages there is not any overshoot, undershoot, dip or abnormal behaviour during the power supply start-up phase. The power supply has been checked over the entire input voltage range with same positive results as in the above figures.

4.3 Power-down

Even at turn off the transition is clean, without any abnormal behaviour like restart or glitches on both the auxiliary and the output voltages. This is still provided by the HV start-up circuitry that controls the power down sequence: at converter power-down the system loses regulation, then Vcc drops and IC activity is stopped as it falls below the UVLO threshold (8.7V typ.). To prevent restart attempts of the converter and to ensure monotonic output voltage decay at power-down an internal logic re-enables the HV current source only if the Vcc voltage goes below the threshold Vccrest located at about 5V. Thus, the HV generator can restart but, if Vin is lower than Vinstart, the HV generator is disabled.

Figure 16. and Figure 17. also the hold-up time at both nominal mains voltages is measured 115 Vac the input Elcap stores enough energy to keep the regulation for 20mS at full load. After that the converter loses regulation and the output voltages drop. The measured time is enough to provide the required immunity of the Set-Top box against standard mains dip or short interruptions tests, required by the standard rules such as the IEC1000 and protecting the unit

Figure 14. at 115 VAC - 60Hz Figure 15. at 230 VAC - 50Hz

CH1: VDDCH2: VC19 (Vaux)

CH3: +12 VoutCH4: +1.8 Vout


17/31

against lost of channel tuning or video disturbances while the end user is watching TV or recording programs.

4.4 Short-Circuit Tests

An important functionality of any power supply is the capability to survive in case of load short circuit and to avoid any consequent failure. Additionally, the power supply must be compliant with safety rules, which require that, in case of fault, no component will melt or burn-out. The SMPS protection is another issue of power supplies. Sometimes it is easy to find circuits with good protection capability against shorts of the load but which are not able to survive in case of a very hard short like that of an output electrolytic capacitor or of a rectifier or transformer saturation. Besides, in case of a shorted rectifier the equivalent circuit changes and the energy is delivered even during the on time, as in forward mode. In case of a short, the voltage at pin Isen exceeds the VISENdis threshold (Hiccup-mode OCP level) and the controller stops the operation, so avoiding the destruction of the components at primary side. The controller remains in off-state until the voltage across the Vcc pin decreases below the UVLO threshold. It will then try to restart without success until the secondary short is removed. This provides a low frequency hic-cup working mode, preventing the power supply from being destroyed.

The board has been tested over the entire input voltage mains range. Two critical circuit parameters, the Vds and the output current have been checked during short circuit tests. In all conditions the measured drain voltage is always below the BVDSS, while the mean value of the output current has a value lower or close to the nominal one, therefore preventing the component damage. As indicated by the waveforms in Figure 18. and Figure 19., once an output is shorted, the circuit begins to work in hic-cup mode, keeping the mean value of the current at levels sustainable by the component rating. Because the working time and the dead time are imposed by the charging and discharging time of the auxiliary capacitor C19, thanks to the L6668 current source and related circuitry, the on-off periods are independent from the input mains voltage, contrary to controller using a passive start-up circuitry where the on time

Figure 16. at 115 VAC - 60Hz Figure 17. at 230 VAC - 50Hz

CH1: VDDCH2: VC19 (Vaux)

CH3: +12 VoutCH4: +1.8 Vout


18/31

duration increases significantly with the mains voltage. The auto-restart at short removal is correct in all conditions.

Figure 20. and Figure 21. show the short circuit waveforms shorting the 3.3V and the 5V output. Like the 12V output voltage the controller keeps under control the circuit preventing in all conditions the power supply from catastrophic failures. Even the 1.8V output is well protected against shorts, in fact figures Figure 22. and Figure 23. are relevant to a short at minimum and maximum mains voltage. Because the coupling between the 1.8V and the auxiliary windings is poor, to help the circuit enter in burst mode the circuitry based on Q2 and

Figure 18. 12V OUTPUT SHORT at 90 VAC Figure 19. 3.3V OUTPUT SHORT at 265 VAC

Figure 20. 3.3V OUTPUT SHORT at 265 VAC Figure 21. 5V OUTPUT SHORT at 265 VAC

CH1: DRAIN VOLTAGECH2: VPIN12(Vcc)

CH3: 12 VoutCH4: ISHORT CIRCUIT


CH3: 3.3 VoutCH4: ISHORT CIRCUIT


CH3: 5 VoutCH4: ISHORT CIRCUIT


19/31

Q3 has been added. This circuit senses the output voltage and drive the circuit to work in hic-cup mode if the 1.8V output voltage disappears

4.5 Over Voltage Protection

A protection that all power supplies must have is that against the failure of the feedback circuitry. If this occurs, the SMPS output voltages can get high values, depending on the load on each output and the transformer coupling between the windings. Consequently, the rectifiers and the output capacitors are overstressed and they can be destroyed. To avoid this SMPS failure, a dedicated low-cost circuit has been added that senses the auxiliary voltage, thus providing a protection threshold not as dependent on the mains voltage or the load. This signal is connected to the L6668, pin 7 (DIS), dedicated to a latched protection of the circuit, as needed to properly protect the power supply from OVP or OTP. The IC pin #7 is the non-inverting input of a comparator having the inverting input internally referenced to 2.2V (typ.). As the voltage on the pin exceeds the threshold, the L6668 stops the operation and its consumption is reduced to a low value. The status is latched until the Vcc goes below the UVLO threshold. To remain in the latch status, the L6668 internal HV generator is activated periodically so that the Vcc oscillates between the start-up threshold VccON and VccON - 0.5V. The SMPS can restart after the disconnection of the converter from the mains and the Vcc pin decreases below the UVLO threshold. Figure 24. depicts the circuit behaviour described above. During a feedback loop failure, the board OVP circuitry voltage intervention threshold allows the 12V output to rise up to 16.8V at 115Vac, and doesn't change significantly with the input voltage or the load.

Figure 22. 1.8V OUTPUT SHORT at 90 VAC Figure 23. 1.8V OUTPUT SHORT at 265 VAC


CH3: 1.8 VoutCH4: ISHORT CIRCUIT


20/31

Figure 24. OVP at 115 VAC - 60Hz

CH1: VPIN7 (OVP)CH2: VC19(Vaux)CH3: +12 Vout

AN2242 5 Conducted Noise Measurements (Pre-Compliance Test)

21/31

5 Conducted Noise Measurements (Pre-Compliance Test)

The following pictures are the conducted noise measurements at full load and 230Vac mains voltage, made on phase and neutral line with Peak detection. The limits shown in the diagrams are the EN55022 CLASS B ones, which is the most common rule for video domestic equipments, such as Set-Top boxes. As clearly visible in the diagrams there is a good margin of the measures with respect to the limits.

Figure 25. Conducted Noise Measurements - Phase A

Figure 26. Concucted Noise Measurements - Phase B

6 Thermal Measures AN2242

22/31

6 Thermal Measures

In order to check the design reliability, a thermal mapping by means of an IR Camera was done. Here below the thermal measures of the component board side at nominal input voltage are shown. Some pointers visible on the pictures have been placed across key components or components showing high temperature. The correlation between measurement points and components is indicated on the right of both figures.

TAMB = 28 degC for all measures

Figure 27. 115Vac-Max Load

Figure 28. 230Vac-Max Load

The thermistor, bridge and output diodes temperature rise are compatible with the reliable operation of the components. Resistor R7 may need a bigger package to decrease its thermal resistance. All other components of the board are working within the temperature limits assuring a reliable long term operation of the power supply.

A 83.70°C E= 0.95 R1 (NTC) B 58.29°C E= 0.95 D1 (BRIDGE) C 116.60°C E= 0.95 R7 (CLAMP) D 75.67°C E= 0.95 T1 (WINDING) E 73.64°C E= 0.95 T1 (FERRITE) F 98.34°C E= 0.95 D2 G 73.85°C E= 0.95 D10 H 69.91°C E= 0.95 D12

A 94.63°C E= 0.95 R7 (CLAMP) B 69.85°C E= 0.95 T1 (WINDING) C 67.62°C E= 0.95 T1 (FERRITE) D 97.50°C E= 0.95 D2 E 76.15°C E= 0.95 D10 F 69.29°C E= 0.95 D12

AN2242 7 Part List

23/31

7 Part List

Table 7. Part List

DesPart Type/

Part ValueDescription Supplier

C10 470uF-25V YXF ALUMINIUM ELCAP - YXF SERIES - 105°C RUBYCON


C12 100N - 50V CERCAP - GENERAL PURPOSE AVX





C18 220N-X2 X2 FILM CAPACITOR - R46-KI 3220 00 L2M ARCOTRONICS


C20 82uF-400V SMH400VN82xx22A - ALUMINIUM ELCAP - SMH SERIES - 85°C

NIPPON CHEMICON

C21 10N-400V R66MD2100AA6-K - METALLIZED POLYESTER FILM CAP.

ARCOTRONICS

C22 1000uF-16V YXF

ALUMINIUM ELCAP - YXF SERIES - 105°C RUBYCON

C23 1000uF-16V YXF






C30 2N2 - 50V CERCAP - GENERAL PURPOSE AVX




C34 220P - 50V CERCAP - GENERAL PURPOSE AVX

C35 2200uF-16V YXF


C36 470P - 50V CERCAP - GENERAL PURPOSE AVX

C37 3N3 - 50V CERCAP - GENERAL PURPOSE AVX




7 Part List AN2242

24/31


C5 DE1E3KX222M 2N2 - Y1 SAFETY CAP. MURATA

C7 1N0 - 200V 200V CERCAP - GENERAL PURPOSE KEMET


D1 2W06G SINGLE PHASE BRIDGE RECTIFIER VISHAY

D10 STPS10L60FP POWER SCHOTTKY RECTIFIER STMicroelectronics

D12 1N5821 LOW DROP POWER SCHOTTKY RECTIFIER STMicroelectronics

D13 LL4148 FAST SWITCHING DIODE VISHAY

D16 LL4148 FAST SWITCHING DIODE VISHAY

D2 STPS8H100FP HIGH VOLTAGE POWER SCHOTTKY RECTIFIER STMicroelectronics

D5 BAV103 FAST SWITCHING DIODE VISHAY

D6 BAV103 FAST SWITCHING DIODE VISHAY

D7 STTH1L06U ULTRAFAST HIGH VOLTAGE RECTIFIER STMicroelectronics

D9 STPS1L60A LOW DROP POWER SCHOTTKY RECTIFIER STMicroelectronics

F1 FUSE T2A FUSE 2 AMP. TIME DELAY WICKMANN

J1 MKDS 1,5/ 2-5,08

PCB TERM. BLOCK, SCREW CONN., PITCH 5MM - 2 WAYS

PHOENIX CONTACT

J2 MPT 0,5/ 8-2,54 PCB TERM. BLOCK, SCREW CONN., PITCH 5MM - 8 WAYS

PHOENIX CONTACT

L1 HF2430-203Y1R0-T01

COMMON MODE CHOKE COIL TDK

L2 2u7-ELC08D POWER INDUCTOR PANASONIC


L4 22uH-RCH654 POWER INDUCTOR SUMIDA


Q1 STP4NK60ZFP N-CHANNEL POWER MOSFET STMicroelectronics

Q2 BC847B SMALL SIGNAL NPN TRANSISTORS STMicroelectronics




R1 NTC_10R S236 NTC RESISTOR P/N B57236S0100M000 EPCOS

R10 10K SMD STANDARD FILM RES - 1/8W - 1% - 100ppm/°C BC COMPONENTS




Table 7. Part List

DesPart Type/


AN2242 7 Part List

25/31

R15 6K2 SMD STANDARD FILM RES - 1/4W - 5% - 250ppm/°C BC COMPONENTS


R17 82R SMD STANDARD FILM RES - 1/8W - 5% - 250ppm/°C BC COMPONENTS

R18 0R47 SFR25 AXIAL STANDARD FILM RES - 0.4W - 5% - 250ppm/°C

BC COMPONENTS

R19 0R47 SFR25 AXIAL STANDARD FILM RES - 0.4W - 5% - 250ppm/°C

BC COMPONENTS

R2 3R9 PR01 AXIAL STANDARD FILM RES - 1W - 5% - 250ppm/°C

BC COMPONENTS











R30 270R SMD STANDARD FILM RES - 1/8W - 5% - 250ppm/°C BC COMPONENTS






R4 4K7 SFR25 AXIAL STANDARD FILM RES - 0.4W - 5% - 250ppm/°C

BC COMPONENTS



R7 33K-2W PR02 AXIAL STANDARD FILM RES - 2W - 5% - 250ppm/°C

BC COMPONENTS

R8 33R PR02 AXIAL STANDARD FILM RES - 2W - 5% - 250ppm/°C

BC COMPONENTS

T1 SRW28LEC-E01 H117

POWER TRANSFORMER TDK

U2 L6668 PRIMARY CONTROLLER STMicroelectronics

Table 7. Part List

DesPart Type/


7 Part List AN2242

26/31

U3 SFH617A-4 OPTOCOUPLER INFINEON

U4 TS2431ILT PROGRAMMABLE SHUNT VOLTAGE REFERENCE STMicroelectronics

HS1 LS220 Q1 HEAT SINK ABL ALUMINIUM COMP.

HS2 507302 D2 HEAT SINK AAVID THERMALLOY

HS3 507302 D10 HEAT SINK AAVID THERMALLOY

Table 7. Part List

DesPart Type/


AN2242 8 PCB Layout

27/31

8 PCB Layout

Figure 29. Silk Screen -Top Side

Figure 30. Silk Screen -Bottom Side

Figure 31. Copper Tracks

9 Transformer Specification AN2242

28/31

9 Transformer Specification

APPLICATION TYPE: Consumer, Home Appliance

TRANSFORMER TYPE: Open

WINDING TYPE: Layer

COIL FORMER: Horizontal type, 6+6 pins

MAX. TEMP. RISE: 45°C

MAX. OPERATING AMBIENT TEMP.: 60°C

MAINS INSULATION: ACC. WITH EN60065

9.1 Electrical Characteristics

CONVERTER TOPOLOGY: Flyback, CCM/DCM Mode

CORE TYPE: EER28L - PC40 or equivalent

TYPICAL OPERATING FREQ.: 65kHz

PRIMARY INDUCTANCE: 910 µH ±10% at 1kHz - 0.25V (1)

LEAKAGE INDUCTANCE: 15 µH MAX at 100kHz - 0.25V (2)

MAX. PEAK PRIMARY CURRENT: 1.65 Apk

RMS PRIMARY CURRENT: 0.65 ARMS

Note: 1 Measured between pins 1-3

2 Measured between pins 1-3 with all secondary windings shorted

Figure 32. "Transformer Electrical Diagram

1

3

PRIM

5

6

AUX

+1.8

7

+3.3

10

9

11

+5V

12

+12V

8

AN2242 9 Transformer Specification

29/31

Table 8. Winding Characteristics

Figure 33. Winding Position

Note: Primaries A & B are in parallel

9.2 Mechanical Aspect

MAXIMUM HEIGHT FROM PCB: 30mm

COIL FORMER TYPE:HORIZONTAL, 6+6 PINS (PINS #2 and #4 ARE REMOVED)

PIN DISTANCE: 5mm

ROW DISTANCE: 30 mm

PINS #3 and #4 ARE REMOVED

EXTERNAL COPPER SHIELD: 12mm WIDTH

9.3 Manufacturer

TDK Electronics Europe - Germany

Transformer P/N: SRW28LEC-E01H117

PINS WINDINGO/P RMS

CURRENTNUMBER OF

TURNSWIRE TYPE

3-1 PRIMARY - A 0.32 ARMS 95 G2 - 2X Φ 0.23mm

12-11 12V 2.3 ARMS 9 G2 - 3X Φ 0.45mm

11-10 5V 3 ARMS 7 SPACED G2 - 3X Φ 0.45mm

9-8 3.3V 0.6 ARMS 2 G2 - 3X Φ 0.45mm

8-7 1.8V 2.1 ARMS 3 G2 - 3X Φ 0.45mm

3-1 PRIMARY 0.32 ARMS 95 G2 - 2X Φ 0.23mm

5-6 AUX 0.05 ARMS 17 SPACED G2 - Φ 0.23mm

COIL FORMER

INSULATING TAPE1.8V – 3.3V

5V12V

PRIMARY - A

PRIMARY - BAUX

44 44

10 Revision History AN2242

30/31

10 Revision History

Date Revision Changes

05-Dec-2005 1.0 First edition

AN2242 10 Revision History

31/31

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequencesof use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is grantedby implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subjectto change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are notauthorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.

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AN2242 APPLICATION NOTE - STMicroelectronics€¦ · Reference design: high performance, L6668-based flyback converter for Set-Top boxes and PVRs Introduction This document describes